1. Field of the Invention
The invention relates to the field of optical recording apparatus and more particularly to lasers for use therein and their associated optics.
2. Brief Description of the Prior Art
The apparatus according to the prior art is shown in FIG. 1. A gas laser 10 projects a beam 12 through acoustic optical modulator 14, a half wave plate 16, a diffraction grating 18, which splits the beam into three beams as better illustrated in FIG. 2 comprised of a central read or write beam 26, a read before write beam 22 and a read after write beam 24. The read or write beam 20 is intended to be centered on a recessed track 28 (FIG. 2) on an optical recording surface 30 while the read before write beam 22 follows the 1eft edge of track 28 and the read after write beam 24 follows the right edge of track 28. Again, referring to FIG. 1, these three beams, after having been diffracted, pass through a field lens 48, a polarizing beam splitter 50, a quarter wave plate 52 and an objective lens 54, which focuses the beams onto the optical recording surface. The beams reflect off the recording surface 30, pass again through the objective lens 54 and the quarter-wave plate 52, are reflected by the polarizing beam splitter toward a cylindrical lens 32 which focuses the respective read before write beams and read after write beams on to track signal detectors 34 and 36, and the main beam on to focusing signal quad detector 60, which also produces the read signal. A portion of the read or write beam 20 is reflected by the polarizing beam splitter into a level detector 56 which provides a feedback through power level control 58 to a drive 38 which in turn controls the power applied to the acoustic optical modulator 14.
Conventionally, the power of the write beam is an order of magnitude greater than the power of the read beam. Typically, therefore, the gas laser 10 is operated at one power level for read operations and a second power level for write operations. This, likewise, causes the read before write and read after write beams 22, 24, derived as they are from the read or write beam 20, to vary in the same ratio of power.
Track signal detectors 34 and 36 operate by discriminating between small changes in reflected intensity as the corresponding beams on track 28 cross a track boundry as shown in FIG. 2. The variations in signal power detected by the track signal detectors 34 and 36 are in turn fed into a servo system (not shown) which maintains the optical system focused on track center.
One can readily see the engineering problem posed by having the read before write and read after write beams 22 and 24 respectively operating at two magnitudes of power an order of magnitude apart, where track signal detectors 34 and 36 are attempting to distinguish between small changes in power. It is the purpose of this invention to overcome this problem.